Heart formation involves a complex series of events in which cells from numerous sources, including the primary and secondary heart fields, neural crest, epicardium and endocardium must interact in specialized ways to form the complex structures of the heart, such as the ventricular chambers, valves and coronary arteries. Our laboratory has discovered four families of basic helix-loop-helix (bHLH) transcription factors (Hands, Hrts, MyoR/capsulin and Twist) that play key roles in multiple steps of cardiac growth and development. The overall goals of this project are to use these bHLH genes as entry points into the developmental circuits that control heart formation and function. We intend to continue to define the roles of bHLH genes in cardiovascular development through lineage analyses and loss-of-function studies in vivo and mechanistic studies in vitro. We will also extend recent observations implicating several cardiovascular bHLH genes in myocardial remodeling, repair, and neovascularization in response to injury. As a complement to our primary focus on mammalian cardiovascular development, we have generated transgenic flies that express green fluorescent protein (GFP) under control of cardiac-specific regulatory elements of the Drosophila Hand gene, which, like the mammalian Hand genes, serves as an early marker of cardiac lineages. These flies have enabled us to undertake an unbiased search for genes that regulate cardial cell specification, differentiation, patterning, growth, morphogenesis and contractility of the heart. The cloning of these mutant genes and eventual elucidation of their functions should provide new insights into the fundamental circuitry of cardiac development and reveal candidate genes for further exploration in mammalian heart development and congenital heart disease. PUBLIC HEALTH RELEVANCE: Abnormalities in heart formation give rise to congenital heart disease, the most common form of human birth defects. Our laboratory has discovered proteins that play key roles in cardiac development and will use these proteins to elucidate the developmental events that control normal heart formation and function. Understanding the function of these proteins will reveal new insights into the fundamental circuitry of cardiac development and provide approaches to combat congenital heart disease.